1. Field of the Invention
This invention relates to coaxial magnetic induction accelerators. More specifically the invention relates to hypervelocity projectile accelerators which create a traveling magnetic wave behind the projectile but which do not require the use of sliding contacts or multiple triggered switches. In particular the invention relates to induction accelerators in which the inductive and resistive parameters relating to the excitation of an elongated stator coil are selected to obtain a traveling magnetic field gradient in response to a pulsed DC power source, without the use of active circuit elements.
2. Description of the Related Art
Thermodynamic guns are widely used and generally understood in a broad context. In an ordinary thermodynamic gun, a propellant burns to generate high pressure gas that pushes a projectile down a bore. While thermodynamic guns are used in many applications besides weapons--for example scientific and industrial applications--their use is somewhat limited because of the maximum velocities attainable. Physical limitations limit the projectile from such thermodynamic guns from reaching velocities much greater than two kilometers per second.
Electromagnetic guns have been widely investigated since World War II as an alternative to thermodynamic guns because of the possibilities of achieving projectile hypervelocities (greater than two kilometers per second). Hypervelocity guns and launchers ar under development for a wide range of applications, including anti-missile systems for strategic defense, impact fusion for nuclear energy production, and launching systems for satellites and spacecraft.
The development of electromagnetic guns has focused mainly upon two different classes of devices, the so-called "railguns," and the magnetic induction accelerators In either case a moving armature is propelled by a magnetic field linking it with stationary electrical conductors in the gun or launcher. In a railgun the stationary conductors are provided by a pair of elongated parallel-spaced rails, and the armature is disposed between the rails and electrically shorts the rails together so that the rails and the armature form an electrical circuit to a power source connected to the rails at a breech end of the gun. In a magnetic induction accelerator, a power source also excites stationary conductors and an electrical current also flows through the armature, but the currents in the stationary conductors and in the armature are not directly linked; instead, the two circuits are indirectly linked by magnetic induction. The degree of coupling between the stationary conductors or "stator" and the armature in a magnetic induction accelerator is quantified by a parameter known as the "mutual inductance" of the stator and armature circuits
The railgun, being the simplest of the electromagnetic projectile accelerators, has enjoyed the most attention and success. In the early electromagnetic railguns, now known as "solid armature" railguns, the projectile was used as the armature. However, it was soon found that at high speeds around one kilometer per second, the rails and projectile were substantially damaged, possibly as a result of ohmic heating and/or internal forces. Further, increases in current flow tended to only increase rail and projectile gouging without an increase in projectile velocity. Thus, projectile velocities in excess of one kilometer per second were not practically attainable for "solid armature" railguns
In the early 1970's, R. A. Marshall, J. P. Barber, and others at the Australian National University, Canberra, Australia, developed railguns using plasma armatures which could obtain hypervelocities and could make efficient use of high current, pulsed power supplies, such as homopolar generators. See, for example, S. C. Rashleigh and R. A. Marshall, "Electromagnetic Acceleration of Macroparticles to High Velocities," 49 J. App. Phys. 2540 (Apr. 1978). In recent years, however, research has revealed numerous problems associated with very high current plasma armatures. At the high currents necessary to obtain hypervelocities, rail erosion and metallic deposits from the plasma armature require the gun to be reamed or rebuilt after one or two shots. In this regard, plasma armature railguns require a sealed bore capable of withstanding the substantial electromagnetic forces generated during firing; the gaskets, seals and insulator materials associated with such bores have been a significant problem.
The application of plasma armature railguns is also constrained due to the fact that the projectile is accelerated using base pressures. Base pressure acceleration (such as is also used in thermodynamic guns) places severe design limits on the projectile. The projectile, for example, must be able to withstand the extreme temperature and pressure exerted at its base by the plasma armature.
A magnetic induction projectile accelerator known as the "coaxial induction" accelerator or ".theta.gun" has been considered as a solution to the problems of the plasma armature or sliding contacts of the railgun. The .theta. gun has multiple coaxial stator coils for centering and driving a tubular copper projectile. In addition, the .theta. gun applies the propelling force along the entire length of the projectile. This has been said to allow much greater acceleration of large "fineness ratio" (i.e , large energy/cross section) projectiles for a given barrel pressure, allowing much shorter barrels for military application. See, for example, Burgess et al., "The Electromagnetic .theta. Gun and Tubular Projectiles", Sandia Nat. Lab. Report No. SAND80-1988.
Extensive experimental and theoretical analysis of the .theta. gun is included in Burgess et al., "The Theta Gun, a Multistage Coaxial, Magnetic Induction Projectile Accelerator", Sandia Nat. Lab. Report No. SAND85-1881 (November 1985). On page 59 the proclaimed advantages of the multistage coaxial magnetic induction mass accelerator are said to be that it is readily staged to become a distributed energy-input system, lack of physical contact between accelerator and projectile, high efficiency, simplified force containment due to its coaxial nature, and higher inductance gradient than a railgun. On page 60 the disadvantages are said to be that the accelerator must be staged, current pulses to each stage must be precisely synchronized, fast switching of high-voltages is required, and switching must be duplicated for each stage. Page 64 says that in the case of rotating machinery power supplies, intermediate power conditioning is required to produce current pulses of rise time short compared to the projectile transit time through a coaxial stage (compared to a railgun), and this power conditioning is very wasteful of energy. Page 64 says that switching duplication is a self-evident, unqualified disadvantage, particularly in the case of a many-staged system with high-velocity projectiles where power conditioning is required. Page 65 further says that since the velocity increase per stage is small, many stages are required to achieve high projectile velocity, and this is very disadvantageous given the complexity of each stage.